1. Field of the Invention
The invention relates to a method for grinding hot, wet raw material according and to a device for grinding hot, wet raw material.
The invention particularly applies to the cement industry for grinding and simultaneously drying raw materials, such as cement clinker, slag and aggregates.
2. Related Art
It is known that for such mill drying processes vertical mills with integrated or mounted classifiers, particularly Loesche-type air-swept roller mills, are particularly suitable. Grinding systems with vertical mills can be operated in compound with rotary kilns and in addition the waste gases from the heat exchanger and clinker cooler process can be used for mill drying and, for the pneumatic conveying of the milled material.
DE 198 36 323 C2 describes methods and plants for the production of cement using a vertical mill.
Besides the mill drying of raw powder in an air-swept roller mill, DE-AS 23 61 060 discloses cooling grinding of cement clinker in an air-swept roller mill downstream of a rotary tubular kiln.
U.S. Pat. No. 4,597,537 discloses a vertical mill having an integrated classifier and a mill housing constructed in a defined manner. For improving classifier efficiency, fitted elements are located in the vicinity of the upper grinding area and upstream of the classifier and influence the grinding material-fluid flow. In the vicinity of the classifier there are also tangential gas supplies with which the gas flow in the classifier is regulated. In the wall of the grit cone is provided a further gas supply for the supply of hot gas, in order to bring about a separation of fine particles adhering to the coarse particles with respect to the latter and the return thereof to the classifying area.
For the grinding of slag and mixed cements use is advantageously made of modified Loesche-type roller mills, which can be referred to as 2+2 or 3+3 roller mills. In these vertical mills use is made of roller pairs, in each case comprising a precompressing roller or S-roller (slave roller) and a grinding roller, or M-roller (master roller) (EP 406 644 B1).
When grinding raw material, such as cement clinker, slag and aggregates, the raw material components have different moisture contents and/or different temperatures. Hot cement clinker, which can have temperatures of 120° C. and higher, is ground in cement plants on increasing the capacity of the rotary kilns and when the existing capacity of the clinker cooler is not adequate and/or if the clinker storage in silos or outside storage does not provide an adequate aftercooling time up to the time of grinding, which can particularly arise with on-line production.
When grinding hot cement clinker in vertical mills with slag and wet aggregates the necessary cooling and drying gases deteriorate in their function as carrying gases. Drying gas or hot gas must be supplied at an adequately high temperature and volume, so that the drying process can take place quickly during grinding. The cooling gas, particularly fresh air, is admixed in regulated manner to the hot gas at corresponding feed temperatures, particularly of the cement clinker or slag.
In order to achieve the desired gas and product temperature downstream of the classifier, it is often necessary to inject water into the grinding chamber, e.g. into the coarse material circulation zone. The disadvantages are an increased energy demand due to the necessary cooling water evaporation, a deterioration of the cement quality and increased wear to the grinding parts, together with an increased energy demand at the mill motor.